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Impact of SMT Assembly Process on Tin Whisker Risk Mitigation

Keywords: tin whisker, assembly, Pb-free

Electroplated pure tin or high-tin finishes on Pb-free components remain a reliability concern due to spontaneous tin whisker growth which may cause failures in electronic equipment. To ensure system reliability, component suppliers and system manufactures have placed significant emphasis on component level tin whisker testing using the method recommended by JEDEC. At the board level, several studies have shown that the SMT reflow process is in general beneficial for whisker mitigation compared with loose components. Nonetheless, the effects of various assembly parameters, such as the reflow temperature or paste volume, are not fully investigated. To better understand the effects of assembly process on whisker growth propensity, we have performed JESD-201 tin whisker testing on both SnPb and SnAgCu assemblies built with components from multiple suppliers, various reflow temperatures, and different paste volumes.
Our results indicate that the SMT assembly process in general reduces the occurrence and maximum length of tin whiskers compared with the loose components subjected to the same reflow profile. This reduction is most noticeable in SAC assemblies, but the paste volume does not present a significant correlation to the whisker growth propensity. Occasionally, long whiskers are found in both SnPb and SAC assemblies. This observation suggests that board assembly process is not always effective in whisker risk mitigation; therefore board level testing should always be performed for verification purpose. The growth locations and detailed analysis on these long whiskers will also be discussed.